Method of manufacturing heat-resistant component using metal granules

ABSTRACT

Disclosed is a method of manufacturing a heat-resistant component using granules. More particularly, the method of manufacturing a heat-resistant component includes a step of preparing granules by spraying a mixture including a metal powder and a slurry material into a housing equipped with a disc and rotating the disc; a step of preparing a molded object by compression-molding the granules; a step of preparing a sintered object by sintering the molded object at about 1,000° C. to about 1,600° C.; and a step of adjusting dimensions by cutting the sintered object, wherein the housing is sealed and hot air at about 70° C. to about 200° C. is supplied into the housing.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No. 10-2015-0187154, filed on Dec. 28, 2015 in the Korean Intellectual Property Office, the entire disclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing a heat-resistant component using granules.

DISCUSSION OF THE RELATED ART

Recently, the demand for industrial parts with superior dimensional accuracy and mechanical properties has been increasing.

Such industrial parts can be manufactured using powder metallurgy technology, metal powder injection molding technology, etc. Powder metallurgy is a technology for manufacturing a molded metal object having a predetermined shape by compression molding a metal powder and then sintering the same.

FIG. 1 illustrates a powder metallurgy method. Referring to FIG. 1, the powder metallurgy method may include molding, sintering and cutting steps.

For example, the powder metallurgy method may include a molding step S1 in which a metal powder is mixed with a binder, as a bonding agent, and then a resultant mixture is subjected to compression to make an ingot, followed by manufacturing a product into a designed shape by a cutting or pressing process; a sintering step S2 in which the product, which has undergone the molding step S1, is heated in a sintering furnace; and a cutting step S3 in which the product, which has undergone the sintering step S2, is ground or cut to designed dimensions.

Since a coarse powder with a particle size of about 50 to 200 μm is used in the powder metallurgy method, it is difficult to secure mechanical properties, i.e., density, strength, hardness, etc., and thus, a produced product cannot be applied to a car turbocharger, etc.

In the molding step, mechanical properties may be improved by increasing compressive strength when the metal powder is fed into a mold and compressed. However, in this case, there is a limitation in increasing compressive strength because a mold can be damaged thereby. Accordingly, there is a disadvantage in that a subsequent process, such as forging or heat treatment, is required.

In addition, in the powder metallurgy method, a coarse powder with a particle size of 50 μm to 200 μm should be used to secure moldability and produce a uniform product. However, a powder with this size forms large pores in a molded object during a molding process. Such large pores may reduce the density of a molded object.

Meanwhile, metal powder injection molding technology uses micro metal particles with a size of nothing more than about 5 μm to about 10 μm. When performing a powder metallurgy method, application of micro metal particles, which are used in metal powder injection molding, has been attempted. However, when micro metal particles with this size are applied, the particles are not closely packed in a mold due to cohesive force between the particles, and thus a problem of non-uniform density occurs in a molding process. In addition, the uniformity of a product is deteriorated due to a non-uniform filling amount.

Moreover, since powder fineness is proportional to difficulty in plastic deformation, the fine powder is under a lot of stress, which is a factor that can cause cracks during heat treatment. Furthermore, a fine powder which has a size smaller than a tolerance between molds causes damage to the molds, and thus the fine powder has a lot of problems in application to powder metallurgy.

FIG. 2 illustrates a metal powder injection molding method. Referring to FIG. 2, the metal powder injection molding method may include a kneading step S100, an injection molding step S200, a degreasing step S300, a sintering step S400, and a cutting S500 step.

More particularly, the metal powder injection molding method may include a mixing step in which a metal powder and a binder, as a bonding agent, are mixed in a mixer; an injection molding step in which a resultant mixture, which has undergone the mixing step is injected, into an injection molding machine and then subjected to compression molding to make a product with a designed shape; a degreasing step in which the product, which has undergone the injection molding step, is heated in a degreasing furnace to remove the binder; a sintering step in which the product, which has undergone the degreasing step, is heated in a sintering furnace; and a cutting step in which the product, which has undergone the sintering step, is ground or cut to a designed size.

The degreasing step is provided to secure the fluidity of the metal powder within the injection molding machine. In addition, since the binder, e.g., wax, a polymer, or the like, may remain as carbon upon heat treatment under an inert atmosphere, the binder should be removed by the degreasing step.

In addition, when performing the metal powder injection molding method, a cumbersome procedure of heating from room temperature to 1,000° C. for about 12 hours to about 60 hours exists in the degreasing step. This causes a decrease in productivity and an increase in fuel costs, and, accordingly, production costs increase. Such a method for manufacturing a heat-resistant component that includes the degreasing step has been disclosed in Korean Patent No. 1202462.

In addition, while a linear shrinkage in the powder metallurgy method is about 1% to about 5%, a linear shrinkage in the metal powder injection molding method is about 12% to about 22%. Thus, the metal powder injection molding method exhibits a considerably large linear shrinkage and has a problem that it is difficult to three-dimensionally control linear shrinkage.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, there is provided a method of manufacturing a heat-resistant component using granules.

In an embodiment of the present invention, the method of manufacturing a heat-resistant component includes a step of preparing granules by spraying a mixture including a metal powder and a slurry material into a housing equipped with a disc and rotating the disc; a step of preparing a molded object by compression-molding the granules; a step of preparing a sintered object by sintering the molded object at about 1,000° C. to about 1,600° C.; and a step of adjusting dimensions by cutting the sintered object, wherein the housing is sealed and hot air at about 70° C. to about 200° C. is supplied into the housing.

In an embodiment of the present invention, the metal powder may include about 0.1% to 3% by weight of carbon (C), greater than 0 and less than or equal to about 5% by weight of silicon (Si), greater than 0 and less than or equal to about 15% by weight of manganese (Mn), greater than 0 and less than or equal to about 1% by weight of phosphorus (P), greater than 0 and less than or equal to about 1% by weight of sulfur (S), greater than 0 and less than or equal to about 90% by weight of nickel (Ni), greater than 0 and less than or equal to about 50% by weight of iron (Fe), and greater than 0 and less than or equal to about 50% by weight of chromium (Cr).

In an embodiment of the present invention, an average size of the granules may be about 20 μm to about 200 μm.

In an embodiment of the present invention, an average particle size of the metal powder may be about 0.01 μm to about 50 μm, and a particle size distribution of the metal powder is about 0.001 μm to about 100 μm.

In an embodiment of the present invention, solid loading (S/L) of the mixture may be about 10% by volume to about 45% by volume.

In an embodiment of the present invention, a rotational speed of the disc may be about 4,000 rpm to about 20,000 rpm.

In an embodiment of the present invention, the slurry material may include a solvent and a binder.

In an embodiment of the present invention, the solvent may include one or more of water, hexane, acetone, and alcohol having a carbon number of 1 to 10.

In an embodiment of the present invention, the binder may include one or more of polyvinyl butyral (PVB), polyvinyl alcohol (PVA), wax, and polyethylene glycol (PEG).

In an embodiment of the present invention, the compression molding may be performed under a pressure of about 0.1 ton/cm² to about 10 ton/cm².

Therefore, it is an objective of the present invention to provide a method of manufacturing a heat-resistant component for preparing spherical granules, and to uniformly fill the interior of a mold with the granules.

It is another objective of the present invention to provide a method of manufacturing a heat-resistant component, the method providing superior press moldability and a sintered object with superior mechanical strength.

It is a still objective of the present invention to provide a method of manufacturing a heat-resistant component, the method minimizing linear shrinkage of a product while providing a product with superior surface and internal quality and superior molding density.

It is yet another objective of the present invention to provide a method of manufacturing a heat-resistant component, the method providing superior productivity and having advantages in terms of processing time and energy consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objectives, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing exemplary embodiments thereof in detail with reference to the accompanying drawings, in which

FIG. 1 illustrates a powder metallurgy method;

FIG. 2 illustrates a metal powder injection molding method;

FIG. 3 illustrates a method of manufacturing a heat-resistant component according to an embodiment of the present invention;

FIG. 4 illustrates a method of preparing granules according to an embodiment of the present invention;

FIG. 5 illustrates a method of preparing granules according to an embodiment of the present invention;

FIG. 6 illustrates a molding machine for preparing granules according to an embodiment of the present invention;

FIG. 7(a) is an optical microscope image of a metal powder used in an example of the present invention, and FIG. 7(b) is an optical microscope image of granules prepared according to an example of the present invention;

FIG. 8(a) is an optical microscope image of granules according to an Example for the present invention, FIG. 8(b) is an optical microscope image of granules according to a comparative example for the present invention, and FIG. 8(c) is an optical microscope image of granules according to a comparative example for the present invention;

FIG. 9(a) is an optical microscope image of granules according to an example of the present invention, FIG. 9(b) is an optical microscope image of granules according to a comparative example for the present invention;

FIG. 10(a) is an image of a heat-resistant component according to an example of the present invention, and FIG. 10(b) is an X-ray image of the heat-resistant component;

FIG. 11(a) is an image of a heat-resistant component according to an example of the present invention, and FIG. 11(b) is an image of a heat-resistant component according to a comparative example for the present invention;

FIG. 12(a) is an image of a heat-resistant component according to a comparative example for the present invention, FIG. 12(b) is an image of a heat-resistant component according to an example of the present invention, FIG. 12(c) illustrates a heat treatment result of the heat-resistant component of the comparative example, and FIG. 12(d) illustrates a heat treatment result of the heat-resistant component of the example;

FIG. 13(a) is an electron microscope image illustrating a microstructure of a heat-resistant component according to a comparative example for the present invention, FIG. 13(b) is an electron microscope image illustrating a microstructure of a heat-resistant component according to an example of the present invention, FIG. 13(c) is an electron microscope image illustrating a microstructure of the heat-resistant component of the comparative example which has been subjected to heat treatment, and FIG. 13(d) is an electron microscope image illustrating a microstructure of the heat-resistant component of the example which has been subjected to heat treatment; and

FIG. 14(a) is an electron microscope image illustrating a surface oxidation layer formed on a heat-resistant component according to an example of the present invention which has been subjected to heat treatment in ambient conditions, and FIG. 14(b) is an electron microscope image showing a surface oxidation layer formed on the heat-resistant component according to the example which has been subjected to heat treatment in a continuous annealing furnace.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention are described in detail so that those of ordinary skill in the art can easily carry out the present invention with reference to the accompanying drawings. The present invention may be implemented in various different forms and is not limited to these embodiments. To clearly describe the present invention, a part not related to the description is omitted in the drawings, and the same or similar elements are designated with the same reference numbers in the entire specification.

Method of Manufacturing Heat-Resistant Component Using Granules

The present invention relates to a method of manufacturing a heat-resistant component using granules. FIG. 3 illustrates a method of manufacturing a heat-resistant component according to an embodiment of the present invention. Referring to FIG. 3, the method of manufacturing a heat-resistant component includes a step of preparing granules (S10); a step of preparing a molded object (S20); a step of preparing a sintered object (S30); and a cutting step (S40).

More specifically, the method of manufacturing a heat-resistant component includes a step of preparing granules by spraying a mixture including a metal powder and a slurry material into a housing equipped with a disc and rotating the disc (S10); a step of preparing a molded object by filling a mold with the granules and then compression-molding the granules (S20); a step of preparing a sintered object by sintering the molded object at about 1,000° C. to about 1,600° C. in a sintering furnace and then cooling the same (S30); and a step of adjusting dimensions by cutting the sintered object (S40).

Hereafter, a detailed description of each step of the method of manufacturing a heat-resistant component will be provided below.

Step of Preparing Granules (S10)

This step relates to preparation of granules by spraying a mixture including a metal powder and a slurry material into a housing equipped with a disc and rotating the disc.

FIG. 4 illustrates a method of preparing granules according to an embodiment of the present invention. Referring to FIG. 4, the method of preparing granules may include a step of preparing a mixture (S11), and a drying step (S12).

Step of Preparing Mixture (S11)

This step relates to preparation of a mixture including a metal powder and a slurry material.

Metal Powder

In an embodiment of the present invention, the average particle size of the metal powder may be about 0.01 μm to about 50 μm. In the present invention, the term “size” is defined as a maximum length of a metal powder particle. When metal powder particles with the above average size are applied, spherical granules may be easily prepared, a mold may be uniformly filled with the granules, and a heat-resistant product with superior mechanical properties may be manufactured.

A size distribution of the metal powder may be about 0.001 μm to about 100 μm (90% or more of the powder). A metal powder with this size distribution may be easily prepared into granules.

In an embodiment of the present invention, the metal powder may be a metal powder for a heat-resistant component which is applied to turbocharged diesel and gasoline engines, etc. The metal powder for a heat-resistant component may include about 18% or more of chromium.

In an embodiment of the present invention, the metal powder may include about 0.1 to 3% by weight of carbon (C), greater than 0 and less than or equal to about 5% by weight of silicon (Si), greater than 0 and less than or equal to about 15% by weight of manganese (Mn), greater than 0 and less than or equal to about 1% by weight of phosphorus (P), greater than 0 and less than or equal to about 1% by weight of sulfur (S), greater than 0 and less than or equal to about 90% by weight of nickel (Ni), greater than 0 and less than or equal to about 50% by weight of iron (Fe), and greater than 0 and less than or equal to about 50% by weight of chromium (Cr). When a metal powder within these ranges is used, the heat-resistant component of the present invention may have superior mechanical strength.

For example, the metal powder may include about 0.2 to 0.5% by weight of carbon (C), about 0.75 to 1.3% by weight of silicon (Si), greater than 0 and less than or equal to about 1.5% by weight of manganese (Mn), about 0.2 to 0.3% by weight of molybdenum (Mo), about 24 to 27% by weight of chromium (Cr), about 19 to 22% by weight of nickel (Ni), about 1 to 1.75% by weight of niobium (Nb), and residual iron (Fe) and inevitable impurities. When a metal powder within these ranges is used, the heat-resistant component of the present invention may be superior in mechanical strength.

In an embodiment of the present invention, HK-30 (ASTM standard) may be used as the metal powder. In particular, HK-30 may be used in a high temperature chemical instrument or a port for heat treatment due to superior heat resistance thereof.

In an embodiment of the present invention, solid loading (S/L) of the metal powder may be about 10% by volume to about 45% by volume. Here, S/L is represented as a ratio of a volume of the metal powder to a total volume of the mixture. The metal powder within this range of S/L has superior compatibility and moldability, and thus the metal powder may be easily prepared into the granules. For example, S/L of the metal powder may be about 13% by volume to about 40% by volume.

Slurry Material

The slurry material secures fluidity of the mixture, so that the metal powder may be sprayed into the housing.

In an embodiment of the present invention, the slurry material may include a solvent and a binder.

Solvent

The solvent may be included in the slurry material to secure fluidity and compatibility of the mixture. In an embodiment of the present invention, the solvent may be volatile. In the specification of the present invention, the term “volatile” means that, by drying with hot air which will be described below, a solvent is vaporized at a temperature range of about 70° C. to about 200° C.

In an embodiment of the present invention, the solvent may include one or more of water, hexane, acetone, and alcohol having a carbon number of 1 to 10. When the solvent is used, superior fluidity and compatibility are exhibited and granules may be easily prepared.

In an embodiment of the present invention, as the alcohol having a carbon number of 1 to 10, monohydric alcohols, including methanol, ethanol, butanol, pentanol, hexanol, etc., dihydric alcohols, including 1,2-pentandiol, 1,5-pentandiol, hexanediol, heptanediol, octanediol, decanediol, etc., and trihydric alcohols, including propylene glycol, 1,3-butylene glycol, glycerin, etc., may be used. These alcohols may be used alone or a mixture of two or more thereof.

In another embodiment, the solvent may be included in an amount of about 50 to about 90 parts by volume based on 100 parts by volume of the mixed slurry material. For example, the solvent may be included in an amount of about 60 parts by volume to about 70 parts by volume. For example, the solvent may be included in an amount of about 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89 or 90 parts by volume.

Binders

The binder may be included to form the granules by agglomerating the metal powder. In an embodiment of the present invention, the binder may include one or more of polyvinyl butyral (PVB), polyvinyl alcohol (PVA), wax, and polyethylene glycol (PEG). When the binder is included, the granules may be effectively formed.

In an embodiment of the present invention, the binder may be included in an amount of about 0.01 part by weight to about 5 parts by weight based on 100 parts by weight of the metal powder. When the binder is included within this weight range, the spherical granules may be easily prepared. For example, the binder may be included in an amount of about 0.05 parts by weight to about 5 parts by weight. For example, the binder may be included in an amount of about 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 4 or 5 parts by weight.

In an embodiment of the present invention, the solvent may include ethanol. In an embodiment of the present invention, the binder may include polyvinyl butyral (PVB). When the slurry material including the solvent and the binder is applied, oxidation of the metal powder may be effectively prevented.

In another embodiment of the present invention, the metal powder and the binder may be included in a weight ratio of about 100:0.01 to about 100:6. When the metal powder and the binder are included in this weight ratio, superior compatibility, workability, and moldability are provided and the granules may be easily prepared. For example, the metal powder and the binder may be included in a weight ratio of about 100:0.05 to about 100:2.

FIG. 5 illustrates a method of preparing granules according to an embodiment of the present invention. Referring to FIG. 5, in an embodiment, the mixture may be prepared by injecting a metal powder 10 into a solvent 20, dispersing the same, and injecting a binder 12 there into.

In another embodiment, a slurry to be mixed with the metal powder 10 may be prepared by injecting the solvent 20 into a container and then dissolving the binder 12 in the solvent 20. Alternatively, the mixture may be prepared by introducing the solvent, the binder, and the metal powder into a ball mill accommodated with balls and mixing the same. In addition, the slurry may be prepared using a general mixer other than the ball mill. In an embodiment of the present invention, the mixture may be prepared by mixing the solvent, the binder, and the metal powder in the mixer for 1 to 2 hours.

Drying Step (S12)

The drying step is a step in which granules are prepared by spraying the mixture into a housing equipped with a disc, and drying the solvent while rotating the disc.

FIG. 6 illustrates a molding machine for preparing granules according to an embodiment of the present invention. Referring to FIG. 6, the molding machine 1000 may include a housing 200 equipped with a rotatable disc 300.

For example, the granules may be prepared by spraying the mixture into the housing 200 equipped with the disc 300, and by drying a solvent included in the mixture while rotating the disc 300. In an embodiment of the present invention, the housing 200 is sealed, and hot air may be supplied to the interior of the housing 200 and may be discharged from the housing 200.

Referring to FIGS. 5(d) and (e), the sprayed mixture is formed into agglomerated spherical particles by surface tension during rotation of the disc 300 and the solvent therein is dried. As a result, the granules 100 are formed.

In an embodiment of the present invention, hot air at about 70° C. to about 200° C. is supplied into the housing 200. When the disc is being rotated while supplying hot air with the above condition, a formation rate and a formation ratio of the granules may be superior without decomposition of the binder. When hot air at less than about 70° C. is supplied into the housing, the mixture is not properly dried and thus shapes of the granules are defective, whereby mechanical properties and an appearance of a heat-resistant component may be deteriorated. On the other hand, when hot air at greater than 200° C. is supplied into the housing, the binder in the mixture is decomposed and thus shapes of the granules are defective, whereby mechanical properties and an appearance of a heat-resistant component may be deteriorated.

For example, hot air at about 80° C. to about 150° C. may be supplied. For example, hot air at about 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 160, 170, 180, 190 or 200° C. may be supplied.

In an embodiment of the present invention, a rotational speed of the disc may be about 4,000 rpm to about 20,000 rpm. At this rotational speed, drying is effectively performed, and thus a formation ratio of spherical granules may be superior. For example, the rotational speed of the disc may be about 4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10000, 11000, 12000, 13000, 14000, 15000, 16000, 17000, 18000, 19000 or 20000 rpm.

In an embodiment of the present invention, the average size of the granules may be about 20 μm to about 200 μm. When the granules are formed in this size range, mold filling ability, and mechanical properties of a heat-resistant component may be superior. For example, the average size of the granules may be about 30 μm to about 150 μm. In another example, the average size of the granules may be about 30 μm to about 90 μm. For example, the average size of the granules may be about 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 160, 170, 180, 190 or 200 μm.

Referring to FIG. 6, the mixture may be dispersed in a shape of water droplets from an upper part of the housing 200 to a lower part of the housing 200 (or a lower part of the housing 200 to an upper part of the housing 200) due to centrifugal force generated by rotation of the disc 300. Here, the solvent in the mixture formed in a spherical shape is dried and spherical granules may be formed. In an embodiment of the present invention, a plurality of the metal powder particles are combined by binding force of the binder and cohesive force (van der Waals force) of the metal powder, and thus the granules may be formed in a spherical shape.

Step of Preparing Molded Object (S20)

This step relates to preparation of a molded object by filling a mold with the granules and by compression-molding the granules. In an embodiment of the present invention, a press mold may be filled with the granules to prepare an ingot of a predetermined shape, which is then subjected to compression molding.

In an embodiment of the present invention, the compression molding may be performed under a pressure of about 0.1 ton/cm² to about 10 ton/cm². The size of the granules may be about ten or more times a size of the metal powder. In addition, since the granules have a spherical shape, fluidity, which is affected by gravity, may be superior. Accordingly, the interior of a mold may be uniformly filled with the granules. That is, the comparison of the granules vs. the metal powder may be explained by a case that a mold is more uniformly filled with rice grains than flour.

In an embodiment of the present invention, the molded object may be prepared to conform to the shape of a heat-resistant component by press-molding the granule-filled mold. For example, a mold may be filled with the granules to prepare an ingot with a shape of a block or a circular plate.

In an embodiment, when a mold of a press machine is filled with the granules to prepare an ingot with a predetermined shape, followed by compression molding, a granular shape of the granules is broken and the granules are split into a fine metal powder, whereby the interior of the mold may be uniformly filled with the fine metal powder.

Step of Preparing Sintered Object (S30)

This step relates to preparation of a sintered object by sintering the molded object in a sintering furnace and then cooling the same.

In an embodiment of the present invention, the sintering may be performed at about 1,000° C. to about 1,600° C. In this temperature range, the binder contained in the mixture is easily removed, and thus the sintered object may exhibit superior mechanical strength and surface properties. When the sintering is performed at less than about 1000° C., a reduction in density, and a reduction in surface and mechanical properties of the sintered object may occur due to incomplete sintering. On the other hand, when the sintering is performed at greater than about 1600° C., shape defects occur due to melting and mechanical properties of the sintered object may be deteriorated. For example, the sintering may be performed at about 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350, 1400, 1450, 1500, 1550 or 1600° C.

In an embodiment of the present invention, the sintered object may be prepared by heating the molded object at the temperature described above in a sintering furnace and subsequently cooling the same to room temperature.

The binder may be decomposed and removed during the sintering. In an embodiment of the present invention, when polyvinyl butyral is used as the binder, polyvinyl butyral is easily removed from a sintering furnace, whereby efficiency of the sintering process is superior and mechanical strength and surface properties of the sintered object are superior.

In an embodiment of the present invention, the sintering may be performed in a gas atmosphere including one or more of hydrogen, nitrogen, and argon. When the sintering is performed in the gas atmosphere, an oxidation phenomenon, which is caused by inflow of oxygen, may be prevented. In an embodiment of the present invention, the sintering may be performed in a gas atmosphere of Ar—N₂ or N₂—H₂.

Cutting Step (S40)

The cutting step relates to adjustment of dimensions by cutting the sintered object. In an embodiment of the present invention, the dimensions of the sintered object may be adjusted by cutting the sintered object prepared in the step of preparing a sintered object to a desired size.

Heat-Resistant Component Manufactured by Method of Manufacturing Heat-Resistant Component

According to another aspect of the present invention, a heat-resistant component is manufactured by the method of manufacturing a heat-resistant component using granules.

When the powder metallurgy method of using the granules according to the present invention is applied, the shape of the prepared granules is close to a spherical shape, and the grain size of the granules is about 20 μm to about 200 μm, and the fluidity of the granules is less than about 40 sec/50 g. Accordingly, the granules have properties similar to a powder for powder metallurgy. Therefore, an interior of a mold may be easily filled with the granules.

In addition, when the granulated powder is subjected to press molding, the granulated powder is broken into a fine powder and thus may uniformly fill the interior of the mold. In addition, a relative density may be increased up to about 99% or more after sintering and a heat-resistant component with superior mechanical properties may be manufactured.

In addition, since fine metal powder particles are granulated and agglomerated, followed by being compressed in a molding step, interior uniformity may be increased compared to a case of compressing a metal powder. In addition, due to a relatively large driving force for sintering, a high density may be obtained after sintering. Thus, even though the powder metallurgy method is used, a product having superior mechanical properties compared to a product manufactured by a metal powder injection molding or casting method may be obtained.

In addition, since, as in the metal powder injection molding method, a degreasing process is not necessary, a manufacturing process is simple. Moreover, since heating is not required for degreasing, energy consumption and processing time are reduced. Therefore, productivity may be improved, and production costs may be reduced.

Further, it may be confirmed that a higher molding density results in a low linear shrinkage after sintering. According to the present invention, since a fine metal powder is used and molding is performed by compressing granules, a molding density may be increased compared to an existing powder metallurgy method. In addition, the control of linear shrinkage is easy compared to a metal powder injection molding method, and thus, after molding, a molded object may be sintered to have dimensions close to predetermined dimensions. Therefore, since work is completed by only the cutting process, an effect of reducing processing and production costs is excellent.

Hereinafter, the constitution and functions of the present invention are described in more detail with reference to examples of the present invention. However, the following examples are merely provided as preferred embodiments and, therefore, the present invention is not limited to the examples.

Examples and Comparative Examples Example 1

According to contents shown in Table 1 below, a mixture including a metal powder, polyvinyl butyral and ethanol was prepared. A container with a volume of 1000 ml and a ball mill were used. A mixture was prepared by mixing the metal powder, the polyvinyl butyral, and the ethanol at room temperature for 1 hour according to conditions shown in Table 1 below.

As the metal powder, HK-30 with an average particle size of 5 to 9 μm, manufactured by Parmaco (Fischingen, Switzerland), was used. The HK-30 includes 0.20 to 0.50% by weight of carbon (C), 24 to 27% by weight of chromium (Cr), 19 to 22% by weight of nickel (Ni), 0.75 to 1.30% by weight of silicon (Si), greater than 0 and less than or equal to 1.5% by weight of manganese (Mn), 0.2 to 0.3% by weight of molybdenum (Mo), 1 to 1.75% by weight of niobium (Nb) and residual iron (Fe) and inevitable impurities.

TABLE 1 Mixture Weight (g) Volume (ml) Metal powder (HK-30) 150 19 Polyvinyl butyral 1.5 1.5 Ethanol 80 103 Solid loading (S/L) 15.7%

Granules were prepared by injecting the mixture into a molding machine including a sealed housing that was equipped with a rotatable disc therein as illustrated in FIG. 6, and then supplying 130° C. hot air into the housing while rotating the disc at a rotational speed of 6000 rpm.

A mold was filled with the granules and subjected to compression molding under a pressure of 100 MPa, i.e., about 1.01 ton/cm², thereby manufacturing a molded object with a form of a vane ring used in a turbocharger.

The molded object was sintered at 1,240° C., which is a temperature lower than a temperature at which a liquid phase is formed, for 2 to 3 hours in a high temperature vacuum batch furnace. The sintering was performed under a vacuum atmosphere in the batch furnace and under a hydrogen atmosphere in a continuous furnace to prepare a sintered object. Subsequently, the dimensions of the sintered object were adjusted by cutting, thereby manufacturing a heat-resistant component.

Example 2

A heat-resistant component was manufactured by the same method as in Example 1, except that a mixture was prepared using components and contents summarized in Table 2 below.

TABLE 2 Mixture Weight (g) Volume (ml) Metal powder (HK-30) 375 48 Polyvinyl butyral 5.6 5.6 Ethanol 100 128 Solid loading (S/L) 27.2%

Example 3

A heat-resistant component was manufactured by the same method as in Example 1, except that compression molding was performed under a pressure of 600 MPa, i.e., about 6.12 ton/cm².

Example 4

A heat-resistant component was manufactured by the same method as in Example 2, except that compression molding was performed under a pressure of 600 MPa, i.e., about 6.12 ton/cm².

Comparative Example 1

A heat-resistant component was manufactured by the same method as in Example 1, except that granules were prepared by supplying 65° C. hot air to the housing.

Comparative Example 2

A heat-resistant component was manufactured by the same method as in Example 1, except that granules were prepared by supplying 215° C. hot air to the housing.

Evaluation of Properties of Heat-Resistant Components

(1) Measurement of relative density, linear shrinkage, apparent density and granule recovery ratio: with regard to Examples 1 to 4 and Comparative Examples 1 to 2, relative densities, linear shrinkages, apparent densities (g/cc), and granule recovery ratios (%) of the manufactured heat-resistant components are shown in Table 3 below.

TABLE 3 Relative Linear Apparent Granule density shrinkage density recovery Classification (%) (%) (g/cc) ratio (%) Example 1 97.6 12.2 2.1 90 Example 2 98.1 12.2 2.3 85 Example 3 98 7.5 2.1 90 Example 4 98.2 7.5 2.3 85 Comparative — — Unmeasurable 0 Example 1 Comparative 95 12 1.9 35 Example 2

Referring to Table 3, since, in the cases of Examples 1 to 4, molding was performed after granules had been compressed, a packing density was increased during mold filling, compared to an existing fine powder. Moreover, since the control of linear shrinkage is easy, after molding, a molded object may be sintered to have dimensions close to predetermined dimensions. Therefore, a heat-resistant component may be manufactured by minimally cutting a sintered object.

On the other hand, in the case of Comparative Example 1, a granulated powder was not formed due to the absence of drying, and, in the case of Comparative Example 2, a granule recovery ratio was significantly lowered compared to Examples 1 to 4.

As illustrated below, FIG. 7(a) is an optical microscope image (magnification: 1000×) of the metal powder used in Example 1, and FIG. 7(b) is an optical microscope image (magnification: 1000×) of the granules prepared according to Example 1 of the present invention. Referring to FIG. 7, it may be confirmed that an average particle size of the metal powder is 5 μm to 9 μm, and the granules prepared according to Examples 1 to 3 are prepared as particles having an average size of 30 μm to 90 μm.

FIG. 8(a) is an optical microscope image of granules according to an Example 1, FIG. 8(b) is an optical microscope image of granules according to a Comparative Example 1, and FIG. 8(c) is an optical microscope image of granules according to a Comparative Example 2; In addition, FIG. 9(a) shows an optical microscope image of the granules of Example 1, FIG. 9(b) shows an optical microscope image of the granules of Comparative Example 1. Referring to FIGS. 8 and 9, in the case of Example 2, it may be confirmed that granules are prepared as particles with an average size of 30 to 90 μm, whereas, in the case of Comparative Example 1, shapes of granules are defective due to incomplete drying of a mixture, and, in the case of Comparative Example 2, shapes of granules are defective due to decomposition of a binder.

FIG. 10(a) is an image of a heat-resistant component of Example 1, and FIG. 10(b) is an X-ray image of the heat-resistant component of Example 1. Referring to FIG. 10, it may be confirmed that appearance of the heat-resistant component of Example 1 is superior and an interior of the heat-resistant component has no defect.

FIG. 11(a) is an image of the heat-resistant component of Example 1, and FIG. 11(b) is an image of the heat-resistant component of Comparative Example 1. Referring to FIG. 11, it may be confirmed that the heat-resistant component manufactured according to Example 1 of the present invention has a superior appearance, whereas the heat-resistant component manufactured according to Comparative Example 1 has a defective appearance due to the occurrence of cracks.

(2) Evaluation of heat resistance: Heat resistance of the heat resistant components manufactured according to the present invention was evaluated as described below. In Comparative Example 3, a heat-resistant component was manufactured by casting using the metal powder of Example 1. Then, heat-resisting properties of the heat resistant components, which were manufactured according to Example 1 and Comparative Example 3 respectively, were evaluated by thermally treating at 820° C. for 50 hours.

FIG. 12(a) is an image of the heat-resistant component of Comparative Example 3, FIG. 12(b) is an image of the heat-resistant component of Example 1, FIG. 12(c) illustrates a result of thermally treating the heat-resistant component of Comparative Example 3, and FIG. 12(d) illustrates a result of thermally treating the heat-resistant component of Example 1.

FIG. 13(a) is an electron microscope image showing a microstructure of the heat-resistant component of Comparative Example 3, FIG. 13(b) is an electron microscope image showing a microstructure of the heat-resistant component of Example 1, FIG. 13(c) is an electron microscope image illustrating a microstructure of the heat-resistant component of Comparative Example 3 which has been subjected to heat treatment, and FIG. 13(d) is an electron microscope image illustrating a microstructure of the heat-resistant component of Example 1 which has been subjected to heat treatment.

Referring to FIGS. 12 and 13, it may be confirmed that, when compared to a microstructure of the casted heat-resistant component of Comparative Example 3 (see FIG. 13(a)), a microstructure of the heat-resistant component of Example 1 (see FIG. 13(b)) exhibits a fine structure and superior homogeneity, but does not exhibit a sigma phase.

Meanwhile, the sigma phase causes reductions in heat resistance and erosion resistance of a heat-resistant component. When the sigma phase is formed in a heat-resistant component, the heat-resistant component may be remarkably damaged under a strong acidic atmosphere, such as a nitrogen atmosphere. Accordingly, in a heat-resistant component for a turbocharger, an area ratio of a sigma phase is limited to be less than 2%.

FIG. 14(a) is an electron microscope image illustrating a surface oxidation layer formed on the heat-resistant component according to Example 1 which has been subjected to heat treatment at 900° C. for 500 hours in ambient conditions, and FIG. 14(b) is an electron microscope image showing a surface oxidation layer formed on the heat-resistant component of Example 1 which has been subjected to heat treatment at 900° C. for 500 hours in a continuous annealing furnace.

Referring to FIGS. 14(a) and 14(b), in the case of Example 1, when heat treatment was performed under a vacuum condition, a maximum thickness of a surface oxidation layer was 12.405 μm, and, when heat treatment was performed in a continuous annealing furnace, a maximum thickness of a surface oxidation layer was 15.405 μm. Accordingly, it may be confirmed that the heat-resistant component for a turbocharger according to Example 1 satisfies a thickness limit value, i.e., less than 30 μm, for a surface oxidation layer.

When manufacturing a heat-resistant component according to the present invention, since the size of granules is ten or more times the size of a metal powder, the granules are uniformly packed in a mold, and, after sintering, the density of the heat-resistant component is uniformly formed.

In addition, according to an embodiment of the present invention, since a metal powder is granulated and then is compressed in a molding step, the granulated metal powder is uniformly packed in a mold compared to the case of compressing a metal powder in a powder metallurgy process. In addition, since densification of a structure may be induced during sintering due to use of a fine metal powder with a superior sintering driving force, a relatively high density may be obtained compared to an existing powder metallurgy process.

Accordingly, when manufacturing a heat-resistant component according to the present invention, a heat-resistant component with superior mechanical properties may be obtained as in the metal powder injection molding method.

In addition, a degreasing process, which is required for the metal powder injection molding method, is not required for the present invention, and thus a manufacturing process according to the present invention is simple. Moreover, since heating, which is required for degreasing, is not necessary, productivity is superior and there are advantages in terms of processing time and energy consumption.

It should be understood by those skilled in the art that various changes in form and details may be made herein without departing from the spirit and scope of the invention as defined by the appended claims. 

What is claimed is:
 1. A method of manufacturing a heat-resistant component, the method comprising: preparing granules by spraying a mixture comprising a metal powder and a slurry material into a housing equipped with a disc and rotating the disc; preparing a molded object by compression-molding the granules; preparing a sintered object by sintering the molded object at about 1,000° C. to about 1,600° C.; and adjusting dimensions by cutting the sintered object, wherein the housing is sealed and hot air at about 70° C. to about 200° C. is supplied into the housing.
 2. The method according to claim 1, wherein the metal powder comprises about 0.1 to 3% by weight of carbon, greater than 0 and less than or equal to about 5% by weight of silicon, greater than 0 and less than or equal to about 15% by weight of manganese, greater than 0 and less than or equal to about 1% by weight of phosphorus, greater than 0 and less than or equal to about 1% by weight of sulfur, greater than 0 and less than or equal to about 90% by weight of nickel, greater than 0 and less than or equal to about 50% by weight of iron, and greater than 0 and less than or equal to about 50% by weight of chromium.
 3. The method according to claim 1, wherein an average size of the granules is about 20 μm to about 200 μm.
 4. The method according to claim 1, wherein an average particle size of the metal powder is about 0.01 μm to about 50 μm, and a particle size distribution of the metal powder is about 0.001 μm to about 100 μm.
 5. The method according to claim 1, wherein solid loading (S/L) of the mixture is about 10% by volume to about 45% by volume.
 6. The method according to claim 1, wherein a rotational speed of the disc is about 4,000 rpm to about 20,000 rpm.
 7. The method according to claim 1, wherein the slurry material comprises a solvent and a binder.
 8. The method according to claim 7, wherein the solvent comprises one or more of water, hexane, acetone, and alcohol having a carbon number of 1 to
 10. 9. The method according to claim 7, wherein the binder comprises one or more of polyvinyl butyral, polyvinyl alcohol, wax, and polyethylene glycol.
 10. The method according to claim 1, wherein the compression molding is performed under a pressure of about 0.1 ton/cm² to about 10 ton/cm². 